U.S. patent number 5,202,063 [Application Number 07/834,438] was granted by the patent office on 1993-04-13 for method for making encapsulated liquid crystal material.
This patent grant is currently assigned to Raychem Corporation. Invention is credited to Brackin L. Andrews, Gilbert Garza, William Seeley, Mark F. Wartenberg.
United States Patent |
5,202,063 |
Andrews , et al. |
April 13, 1993 |
Method for making encapsulated liquid crystal material
Abstract
Encapsulated liquid crystal material is made by mixing liquid
crystals, a containment medium, and a carrier medium made of a
water-miscible monohydric alcohol and water in the proportion
between 5:95 and 60:40 weight/weight to form an emulsion; applying
the emulsion onto a substrate and drying the emulsion to remove the
carrier medium.
Inventors: |
Andrews; Brackin L. (Fremont,
CA), Garza; Gilbert (Fremont, CA), Wartenberg; Mark
F. (San Jose, CA), Seeley; William (Newark, CA) |
Assignee: |
Raychem Corporation (Menlo
Park, CA)
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Family
ID: |
24622345 |
Appl.
No.: |
07/834,438 |
Filed: |
February 12, 1992 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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653795 |
Feb 11, 1991 |
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Current U.S.
Class: |
264/4.6; 427/162;
349/89; 349/92; 252/299.1; 516/194; 516/70; 516/75; 516/900;
516/72; 516/68; 252/299.5; 252/299.01 |
Current CPC
Class: |
C09K
19/544 (20130101); G02F 1/1334 (20130101); Y10S
516/90 (20130101); G02F 1/13725 (20130101) |
Current International
Class: |
C09K
19/54 (20060101); G02F 1/13 (20060101); G02F
1/1334 (20060101); G02F 1/139 (20060101); B01J
013/02 (); G02F 001/13 (); C09K 019/00 () |
Field of
Search: |
;264/4.1,4.3,4.6
;252/299.01,299.1,312,314 ;359/51,52,103,106 ;428/402.2
;427/162 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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156615 |
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Oct 1985 |
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EP |
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204537 |
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Dec 1986 |
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EP |
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Primary Examiner: Stoll; Robert L.
Assistant Examiner: Covert; John M.
Attorney, Agent or Firm: Chao; Yuan Burkard; Herbert G.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation-in-part of application Ser. No.
07/653,795, filed Feb. 11, 1991, now abandoned, the disclosure of
which is incorporated herein by reference.
Claims
What is claimed is:
1. A method of making an encapsulated liquid crystal material,
comprising the steps of:
providing liquid crystals and a containment medium selected from
the group consisting of poly(vinyl alcohol) and poly(vinyl alcohol)
copolymers, gelatin, polymethyl vinyl ether/maleic anhydride,
carboxy polymethylene polymer, poly(ethylene oxide), poly(vinyl
pyrrolidone), cellulosic polymers, and natural gums;
providing a carrier medium comprising a water-miscible monohydric
alcohol and water in the proportion between 5:95 and 60:40
weight/weight;
mixing the liquid crystals, containment medium, and carrier medium
to form an emulsion;
applying the emulsion onto a substrate; and
drying the emulsion to remove the carrier medium.
2. A method according to claim 1 in which the liquid crystals are
operationally nematic liquid crystals.
3. A method according to claim 1 in which the liquid crystals
include a pleochroic dye.
4. A method according to claim 1, 2, or 3, wherein the alcohol to
water proportion if between 10:90 and 50:50 weight:weight.
5. A method according to claim 1, 2, or 3, wherein the alcohol is
selected from the group consisting of ethanol, methanol,
isopropanol, n-propanol, and t-butanol.
6. A method according to claim 1, 2, or 3, further comprising the
step of filtering the emulsion.
7. A method according to claim 1, 2, or 3, further comprising the
step of diluting the emulsion with water after the mixing step.
8. A method according to claim 1, 2, or 3, further comprising the
step of removing part of the alcohol from the emulsion by
evaporation after the mixing step.
9. A method according to claim 1, 2, or 3, wherein the containment
medium is poly(vinyl alcohol) and the alcohol is ethanol.
Description
BACKGROUND OF THE INVENTION
This invention relates to a process for making encapsulated liquid
crystal material suitable for use in electrooptical devices.
Many types of liquid crystal devices are known. Among these the
most well known are displays, but other liquid crystal devices
include privacy screens, sunroofs, membrane switches, and shutters.
A preferred type of liquid crystal device employs encapsulated
liquid crystal material, wherein liquid crystals are encapsulated
or dispersed in a matrix or containment medium such as a polymer.
When a voltage corresponding to a sufficiently strong electric
field is applied across the encapsulated liquid crystal material
(the "field-on" condition), the alignment of the liquid crystals is
re-oriented in accordance with the field, so that incident light is
transmitted. Conversely, in the absence of such a voltage (the
"field-off" condition) the alignment of the liquid crystals is
random and/or influenced by the liquid crystal-matrix interface, so
that the liquid crystal material scatters incident light. The
applied voltage at which the liquid crystal material begins to
change from its field-off condition to its field-on condition is
called the threshold voltage. Encapsulated liquid crystal materials
and their use in devices are discussed in Fergason, U.S. Pat. Nos.
4,435,047 (1984), 4,579,423 (1986), 4,605,284, 4,616,903, and
4,707,080; Doane et al., U.S. Pat. No. 4,890,902; West et al., U.S.
Pat. No. 4,685,771 (1987); and Doane et al., U.S. Pat. No.
4,688,900 (1987), the disclosures of which are incorporated herein
by reference.
The size and size distribution of the liquid crystals droplets
contained in the matrix can affect the performance of the liquid
crystal material. When an encapulated liquid crystal material is
prepared, the droplets will be produced in a range of sizes. The
smaller droplets have a higher threshold voltage, so that in a
material having a substantial amount of smaller, submicron sized
droplets, or fines, switching all the liquid crystal droplets to
the field-on condition requires a higher voltage. The result is
that material having a large amount of fines will appear hazy until
such higher voltage is applied, and will not switch sharply between
a nontransmissive state and a transmissive state. An encapsulated
liquid crystal material in which the droplet size distribution is
relatively narrow will also have a sharper turn-on effect, because
more of the droplets will have the same threshold voltage. It is
also taught in Wu et al., U.S. Pat. No. 4,671,618 (1987) that the
switching time of encapsulated liquid crystal material is affected
by the droplet size.
In one method of making encapsulated liquid crystal material, an
emulsion of the containment medium and liquid crystals is initially
produced, optionally together with a carrier medium. The use of
water as a carrier medium is taught in the aforementioned Fergason
U.S. Pat. No. 4,435,047. The emulsion is spread onto a substrate
and allowed to dry, to produce a film or sheet of encapsulated
liquid crystal material. It is desirable that the emulsion be
readily spreadable onto the substrate, to form uniform and
defect-free films. Further, it is also desirable that the carrier
medium be readily volatilized, so that the films dry quickly and
are less likely to incur defects or be contaminated. Otherwise,
there may be redistribution of the droplets, leading to an
inhomogeneous dried film, due to variations in the flatness and/or
surface energy of the substrate.
SUMMARY OF THE INVENTION
An object of this invention is to make encapsulated liquid crystal
material having liquid crystal droplets with narrow size
distribution. Another object of this invention is to make
encapsulated liquid crystals in which the amount of fine droplets
is reduced. Yet another object of this invention is to provide an
improved method of making films of encapsulated liquid crystal
material in which the emulsion is readily coated onto a substrate
and dries quickly to provide high quality films with reduced
numbers of defects.
According to this invention, encapsulated liquid crystal material
is made by:
providing liquid crystals and a containment medium;
providing a carrier medium comprising a water-miscible monohydric
alcohol and water in the proportion between 5:95 and 60:40
weight/weight;
mixing the liquid crystals, containment medium, and carrier medium
to form an emulsion;
applying the emulsion onto a substrate; and
drying the emulsion to remove the carrier medium.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows the effect of the amount of ethanol in the carrier
medium on the surface tension of the carrier medium.
FIG. 2 shows the effect of the amount of ethanol in the carrier
medium on the drop size distribution as a function of mixing
time.
FIG. 3 compares the stabilities of emulsions made with various
amounts of ethanol in the carrier medium.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In encapsulated liquid crystal material, discrete volumes of liquid
crystals are encapsulated, dispersed, embedded or otherwise
contained in a containment medium or matrix. "Liquid crystals"
denotes a composition having liquid crystalline properties, whether
that composition is a single discrete liquid crystalline compound,
a mixture of different liquid crystalline compounds, or a mixture
of liquid crystalline and non-liquid crystalline compounds.
Preferably, the liquid crystals are nematic or operationally
nematic. Other types of liquid crystals, such as smectics and
cholesterics can also be encapsulated by the methods of this
invention.
Liquid crystals have typically elongated molecular shapes, with a
tendency to align or orient themselves with their long molecular
axes parallel to each other. This alignment causes liquid crystals
to be anisotropic, meaning that their measured physical, optical,
and other properties are dependent on the direction of measurement
(parallel or perpendicular to the direction of alignment). Further,
the alignment direction can be influenced by an external stimulus,
such as an electrical or magnetic field, causing the liquid
crystals to exhibit a particular value of a physical characteristic
in one direction when the stimulus is absent, but rapidly switching
to a different value when the stimulus is applied. It is because of
their anisotropy and their ready realignment that liquid crystals
are useful as materials for displays.
The containment medium is preferably a polymeric material which is
soluble in the carrier medium. Specific preferred containment media
include but are not limited to poly(vinyl alcohol) and poly(vinyl
alcohol) copolymers, gelatin, polyelectrolytes such as Gantrez.TM.
(polymethyl vinyl ether/maleic anhydride, from GAF Corporation) and
Carbopole.TM. (carboxy polymethylene polymer, from B. F. Goodrich
Chemical Corporation) poly(ethylene oxide), poly(vinyl
pyrrolidone), cellulosic polymers, natural gums, and the like.
Poly(vinyl alcohol) is a preferred containment medium because of
facility with which it forms emulsions with liquid crystals.
Typically, encapsulated liquid crystal material is substantially
nontransparent in the absence of a sufficient electric field (the
"field-off" state) and substantially transparent in the presence of
a sufficient electric field (or "field-on" state). The electric
field induces a change in the alignment of the liquid crystals, in
turn causing the encapsulated liquid crystal material to switch
from a highly light-scattering (and/or absorbent) state to a highly
non-scattering and substantially transparent state. Generally, it
is preferred that the liquid crystals have a positive dielectric
anisotropy and that the ordinary index of refraction of the liquid
crystals be matched with the refractive index of the containment
medium, while the extraordinary index of refraction is
substantially mismatched therewith. The physical principles by
which such encapsulated liquid crystal material operates is
described in further detail in the aforementioned references,
particularly the patents to Fergason. In those portions of the
encapsulated liquid crystal material to which a sufficient electric
field is applied, the transition from a non-transparent state to a
transparent state occurs, while adjacent areas to which no electric
field has been applied remain non-transparent.
Pleochroic dyes have been mixed with liquid crystals to form a
solution therewith. The molecules of pleochroic dyes generally
align with the molecules of liquid crystals, so that the
application of the electric field affects not only the predominant
alignment of the liquid crystals, but also of the pleochroic dye.
As the extent of the absorption of incident light by the pleochroic
dye depends on its orientation relative tot he incident light, the
application of an external stimulus to a liquid crystal-pleochroic
dye combination also provides a means for the controlled
attenuation of light. Generally, the pleochroic dye is in a
substantially more light absorbing state in the field-off condition
and in a substantially more light transmissive state in the
field-on condition. (Thus, as used herein, the term "liquid
crystals" also means, in context, liquid crystals containing
pleochroic dye dissolved therein.) Pleochroic dyes may be used in
encapsulated liquid crystals to form colored displays. A display
capable of displaying colored images can be formed by depositing
side by side red, blue, and green pixels.
We have discovered that where a solution of a water-miscible
monohydric alcohol and water is used as the carrier medium in
preparing an emulsion of liquid crystals and the containment
medium, encapsulated liquid crystal material having a narrower
distribution of liquid crystal droplets is produced. We believe,
without wishing to be bound by theory, that the effect is due to
the reagglomeration of fines and to the production of droplets of
fairly small size at lower shear stresses due to reduced surface
tension. Further, the lower surface tension of the emulsions makes
them more readily spreadable as uniform, faster drying films.
Suitable water miscible monhydric alcohols include ethanol,
methanol, isopropanol, n-propanol, and t-butanol. A preferred
alcohol is ethanol. The alcohol-water ratio is between 5:95 and
60:40 weight:weight, preferably between 10:90 and 50:50
weight:weight. While infinitely miscible monhydric alcohols are
preferred, partially miscible ones, such a n-, iso- and 2-butanol,
may be used up to the extent of their respective solubilities. At
increasing alcohol levels, there may be some solubilization of the
liquid crystals into the alcohol:water, or some precipitation of
the liquid crystals. Such effects can be avoided by choosing the
appropriate combination of liquid crystals and alcohol in the
appropriate proportions, as can be readily determined
empirically.
Monohydric alcohols are preferred over polyhydric alcohols because
of their higher volality. A polyhydric alcohol, such as glycerol,
would be very difficult to remove because of its low volatility and
high affinity for the containment media such as poly(vinyl
alcohol). (Further, we have found that glycerol does not provide
the particle size distribution improvements observed by us with our
monhydric alcohols.) Thus, the preferred monohydric alcohols are
those whose boiling points at one atmosphere are not significantly
higher than that of water, i.e., below about 120.degree. C. With
more volatile monohydric alcohols, drying the emulsion and removal
of the containment medium at ambient temperatures (i.e., about
20.degree.-30.degree. C.) without special heating techniques which
would be costly and/or damaging the the liquid crystal or
containment medium is feasible.
A ratio of 10% by weight containment medium in the carrier medium
(e.g., 10% (w) poly(vinyl alcohol) in 50:50 (w/w) ethanol:water)
has been found to be preferred. Those skilled in the art will
appreciate that the desirable amount of carrier medium is dependent
on the molecular weight and chemical characteristics of the
containment medium. We have found that mixing the appropriate ratio
of these to produce a mixture of viscosity 100 cps generally
produces a system with which it is convenient to work, but a wide
range of other concentration/viscosity combinations can also
work.
The mixing of the liquid crystals, containment medium, and carrier
medium can be accomplished with equipment such as propeller blade
mixers, homogenizers, and colloid mills. Good results have been
obtained with a Cole-Parmer mixer with 3-prong propeller blades
driven by a permanent magnet DC motor with solid state controllers
which provide a uniform mixing speed regardless of torque.
FIG. 1 shows that the inclusion of ethanol in a carrier medium
comprising 10% poly(vinyl alcohol) lowers its surface tension,
making the resulting emulsion easier to spread on substrates such
as indium tin oxide (ITO) coated polyester or glass. ITO coated
polyester or glass is a preferred substrate because the ITO can
serve as a transparent electrode material for applying the
threshold voltage to the liquid crystal material, while the
polyester or glass provides physical support and protection.
FIG. 2 shows the effect of carrier medium composition on the
polydispersity of the emulsion produced, as a function of the
mixing time. Polydispersity is defined as the volume median
diameter divided by the number median diameter (V/N ratio). A V/N
ratio of 1 corresponds to a monodisperse system. As can be seen,
increasing the ethanol concentration decreases the V/N ratio.
The particle size of emulsions produced can be measured by a
variety of methods, including with a Coulter Counter Multisizer
apparatus and various light scattering techniques. We prefer to use
the Multisizer. The volume, area, and number medians are
calculated, and thence, V/N.
FIG. 3 shows that emulsions based on carrier media having up to at
least 25% ethanol are as stable as those made with water as the
carrier medium. However, emulsions made with a carrier medium of
50:50 ethanol:water show instability, indicating that they are
preferably promptly used. After they are prepared, they can be
diluted with water, to reduce the alcohol concentration and produce
a stable emulsion with a narrow droplet size distribution.
Alternatively, part of the alcohol can be removed, thereby reducing
its concentration, by evaporation, preferably at reduced pressure
using a rotary evaporator.
The droplet size distribution in the emulsion can be further
improved by filtering the emulsion initially produced through a
filter or membrane material. As the use of an ethanol:water carrier
medium produces a relatively small amount of fine droplets, such a
filtration step, which removes the larger droplets, leads to an
emulsion having especially narrow droplet size distribution. A
preferred filter material is Versapor membrane, made by Gelman
Sciences (Ann Arbor, Mich.), in the 1 .mu.m to 5 .mu.m range.
The practice of this invention can be further understood by
reference to the following examples, which are provided by means of
illustration and not of limitation.
EXAMPLE 1
A mixture of 30 g of liquid crystal (ZLI-3401 from Merck GmbH,
Darmstadt, Germany) with 100 g of 10% poly(vinyl alcohol) (Airvol
205, Air Products and Chemicals, King of Prussia, Pa.) solution in
50:50 ethanol:water was prepared by high shear mixing using a
propeller blade mixer (Stir-Pak heavy duty mixer with 1 inch
blades, Cole-Parmer, Chicago, Ill.) at 2300 rpm for 3 minutes.
This procedure produced an emulsion of unusually narrow droplet
size distribution: 1.46 .mu.m by number, 1.77 .mu.m by area, and
1.92 .mu.m by volume, corresponding to a V/N ratio equal to
1.92/1.46=1.32.
EXAMPLE 2
The procedure of Example 1 was repeated, except that the mixing was
performed for 10 min at 1500 rpm, to produce an emulsion with this
droplet size distribution: 1.98 .mu.m by number, 5.66 .mu.m by
area, and 7.27 .mu.m by volume (V/N=7.27/1.98=3.67). This emulsion
was filtered three times through a 3 .mu.m Versapor membrane filter
(Gelman Sciences, Ann Arbor, Mich.) to produce an emulsion with
droplet size distribution: 1.84 .mu.m by number, 2.40 .mu.m by
area, 2.64 .mu.m by volume (V/N=2.64/1.84=1.43).
EXAMPLE 3
A mixture of 7 g of liquid crystal (BDH-AG, British Drug House,
Poole, England) and 70 g of 10% poly(vinyl alcohol) (Airvol 205,
Air Products & Chemicals, King of Prussia, Pa.) solution in
50:50 t-butanol:water was prepared by high shear mixing using a
propeller blade mixer (Stir-Pak mixer with 1 inch diameter blade,
Cole Palmer, Chicago, Ill.) at 6600 rpm for 8 min. The speed was
then reduced to 1500 rpm and 63 g of deionized water was added over
the next minutes, followed by one more minute of mixing to produce
an emulsion with this droplet size distribution: 3.16 .mu. by
number, 12.8 .mu. by area, and 15.5 .mu. by volume (V/N
ratio=15.5/3.16=4.90).
This emulsion was filtered three times through a 3 .mu. Versapor
membrane filter (Gelman Science, Ann Arbor, Mich.) to produce an
emulsion droplet with this size distribution: 1.61 .mu. by number,
1.99 by area, and 2.19 by volume (V/N=2.19/1.61=1.36).
* * * * *